1. Field of the Invention
This invention relates in general to a heat resistant and wear resistant iron-based sintered alloy as the material of a component part which requires heat resistance, wear resistance and corrosion resistance while exhibiting low attacking ability against and high concordance with an opposite contactable member, and more particularly to a material suitable for a valve seat, a valve face and a waste gate valve of a turbocharger for an internal combustion engine.
2. Description of the Prior Art
In recent years, wear of component parts of an engine valve system for an internal combustion engine has become an issue with requirement of increasing engine speed and engine power output, in which particularly valve seats require excellent oxidation resistance, friction characteristics and durability at high temperatures under severe high temperature engine combustion conditions. In this connection, a turbocharger is also required to be formed of a material durable in oxidation and friction at high temperatures.
Thus, since such component parts of the engine are subjected not only to high temperatures but also to friction and hammering, they are required to be formed of a material having high heat resistance, wear resistance and pitting resistance together with concordance with the material of an opposite contactable member. Materials which have been hitherto proposed and put into practical use as the above-mentioned material are heat resistant steel, other heat resistant materials of the crystallized and hardened type, cermets containing ceramic particles, alloys of the dispersion strengthened type, and the like. Additionally, as the material of a valve seat, a sintered material containing molybdenum (Mo) has been proposed, using self-lubrication effect due to higher hardness at high temperatures and oxide film which are resulted from Mo which is not diffused. In this connection, the material of the type wherein Mo is dispersed in iron (Fe) is disclosed, for example, in Japanese Patent Provisional Publication No. 58-71355. Furthermore, alloys in which metal-carbide and/or metal-silicide is crystallized on the surface thereof have been proposed.
However, such conventional heat resistant materials are lower in high temperature hardness and wear resistance. More specifically, the material containing ceramic particles tend to cause abnormal wear owing to lower bonding force between the particle and the matrix, while damaging the opposite contactable member. Fe--Mo compound powder and Mo powder are low in compactibility and in wetting property with the matrix. Accordingly, the material containing these powders tends to break while causing the powder to remove therefrom, thereby attacking the opposite contactable member. Moreover, the carbide and the silicide allow the material containing them to be lowered in oxidation resistance at high temperatures. Thus, the above-discussed conventional materials are not sufficient in a required performance while unavoidably attacking the opposite contactable member and are therefore problematic in practical use.